Rock magnetic properties of sediments from Ceara Rise (Site 929): implications for the origin of the magnetic susceptibility signal

Abstract

This paper presents a rock magnetic study of cyclic deep-ocean sediment from the Ceara Rise in the equatorial Atlantic Ocean off the coast of Brazil. Whole-core magnetic susceptibility data have been used as a proxy for climate change in these sediments. This study is aimed at testing this assumption and at determining the source of the magnetic susceptibility variations. We analyzed hysteresis properties, high-Þeld and low-Þeld susceptibilities, anhysteretic remanent magnetization, and thermomagnetic behavior of 114 sediment samples. Hysteresis measurements show that the magnetic carrier is of pseudosingle domain size. Low-temperature demagnetization data indicate the Verwey transition (diagnostic of magnetite) at 118 K. The magnetite concentration was estimated from the saturation magnetization values and varies between 43 ppm in light, carbonaterich layers and 94 ppm in dark, carbonate-poor layers. The average low-Þeld mass susceptibility is 12.56 × 10 Ð8 m 3 /kg, and the average high-Þeld susceptibility is 4.8 × 10 Ð8 m 3 /kg. The contribution of magnetite to the low-Þeld susceptibility is 63% in carbonate-poor layers and 53% in carbonate-rich layers. Concentration-independent rock magnetic ratios revealed small-scale and large-scale variations in the magnetic grain size. The variations in magnetic properties mirror changes in clay content, which provides excellent evidence that the susceptibility variations resect suctuations in sedimentary input from the Amazon River. The dominant control on Amazon-derived terrigenous sediment is sea level. Therefore, it appears that magnetic susceptibility is a valid climate proxy for the Ceara Rise sediments.